To integrate the optical zoom function in a compact optical system comprising several lenses with fixed focal length, it can be interesting, especially in light of minimizing the thickness of the optical system, to integrate at least one optical device with variable focal length to create major variations in focal length.
Some optical devices ensure a variation in magnification, and others have to be linked to fixed optics or to other devices with variable focal length to ensure zoom function.
For applications with miniature cameras, especially cameras designed for mobile telephony, the aim is to design an optical device with variable focal length which is compact and inexpensive.
A device for working either in a divergent mode or in a convergent mode is particularly advantageous as it benefits from a wider range in variation of focal length.
Different types of devices having variable focal length based on liquid have been developed to respond to this need, based for example on an electrowetting technique [1] or based on liquid crystals [2].
Other solutions are based on the use of deformable membranes subject to fluid pressure, each membrane forming a diopter.
Two membranes are generally employed to boost the optical power of the device.
In these devices, the membranes deform under the effect of the fluid pressure caused by a fluid displacement.
In some applications, each of the two membranes is actuated independently of the other, each membrane being associated with a cavity enclosing the fluid, which is separated from the other by a substrate, and having its own actuation device [3, 4].
The fluid pressure imposed on one of the membranes can be different from the other.
Different configurations of lenses, where the actuators intended to vary focal length are arranged at the periphery of each membrane, are presented in FIGS. 1A and 1B.
FIG. 1A illustrates a bi-convex convergent lens, which can be symmetrical or not.
This device comprises a support 3 to which two deformable membranes 1, 2 are connected in a respective peripheral anchoring area 1c, 2c, and a rigid plate 31 extending between both membranes. Each membrane 1, 2, with the support 3 and the plate 31, defines a respective constant volume of fluid 41, 42. Due to the presence of the rigid plate 31 interposed between the volumes of fluid 41 and 42, the deformations of both membranes are independent of each other.
Each membrane 1, 2 is provided with an actuation device 5, 5′ arranged on a respective intermediate area 1a, 2a between the central part 1b, 2b and the anchoring area 1c, 2c of each membrane.
The actuators 5, 5′ of both membranes 1, 2 deflect towards the membrane and the fluid 41, 42 (in the direction of the arrows) to create the bi-convex configuration illustrated in FIG. 1A.
FIG. 1B corresponds to a bi-concave divergent lens which can be symmetrical or not.
The structure of the device is similar to that of the device of FIG. 1A, the actuation device 5, 5′ of each membrane being capable of deflecting in the direction opposite that of FIG. 1A (in the direction of the arrows) to create such a configuration.
To the extent where both diopters function independently of each other, it is also possible to obtain many other configurations of convergent lenses (convex plane or convergent meniscus) or divergent (concave plane or divergent meniscus).
FIGS. 2A to 2C illustrate another example of an optical device exhibiting a structure similar to that of FIG. 1A, in which the membrane 2 ensures the convergent function (with deflection of the actuation device 5′ in one direction, towards the fluid) and the membrane 1 ensures the divergent function (with deflection of the actuation device 5 in the opposite direction), each membrane being associated with an independent volume of fluid 41, 42. The elements designated by the same reference numerals in FIGS. 1A-1C and 2A-2C are similar.
FIG. 2A illustrates the optical device at rest, taking the example of an initial infinite focal length.
FIG. 2B illustrates the optical device actuated so as to be convergent: for this purpose, the actuation device 5 is not activated, and only the actuation device 5′ is activated so as to flex towards the fluid 42, resulting in a convex plane convergent lens.
FIG. 2C illustrates the optical device actuated so as to be divergent: for this purpose, the actuation device 5′ is not activated, and only the actuation device 5 is activated so as to flex towards the fluid 41, resulting in a concave plane divergent lens.
In other applications, both membranes are coupled by a constant volume of fluid enclosed between said membranes [5-10], both membranes are subject to the same fluid pressure.
FIGS. 3A and 3B illustrate such optical devices. Relative to the device illustrated in FIGS. 1A to 2C, the device in FIGS. 3A and 3B contains no plate separating the volumes of fluid associated with each membrane. The device therefore contains a single constant volume of fluid 4 mechanically coupling the membranes 1 and 2.
In these devices, the actuation devices 5, 5′ of both membranes jointly contribute to modify the fluid pressure applied to each of said membranes.
When the actuation devices 5, 5′ deflect towards the fluid 4, the pressure is increased and both membranes 1, 2 become convex (convergent device, see FIG. 3A).
When the actuation devices 5, 5′ deflect in the direction opposite the fluid 4, both membranes 1, 2 become concave (divergent device, see FIG. 3B).
To produce an optical system which can have variations in positive and negative focal length from a rest position, the actuation devices must function in both directions.
However, such actuation is complex to implement.
In fact, the electrical voltages required for the actuators are typically above 30V (voltage +/−40V is mentioned in [3]).
In existing devices, thick layers of piezoelectric materials are stuck onto each membrane, which involves a complex manufacturing method and imposes a certain number of restrictions on the membrane and on the geometry of the device.
It is however possible to carry out such dual-direction actuation by using MEMS technologies.
Either materials which are capable of deforming in both directions should be used, but these materials (AlN for example) are less effective in terms of actuation, or a bimorphic actuator comprising two layers of PZT ceramic (Lead Titano-Zirconate) should be used.
Given that PZT ceramic has a high manufacturing cost, this latter option is particularly costly.
The device illustrated in FIGS. 2A to 2C can be alternatively convergent and divergent, by using actuators operating in a single direction.
The disadvantage of such a device, which consists of superposing two single-membrane devices, is that it offers variations in focal lengths much less than the variations produced by the devices illustrated in FIGS. 1A-1B and 3A-3B.
In fact, a single diopter is active in the convergent configuration, the other diopter being active in the divergent configuration.